The present invention relates generally to a burner system, and more particularly, to a burner and burner combustion process having very low pollutant emission throughout the burner firing range.
Fuel burners are used in boilers, heaters, and other applications for the conversion of fuel to heat. The heat is then transferred to make hot water, steam, and/or warm air, or to create power, depending upon the application. Burners generally mix fuel and air and then direct the mixture for the purpose of creating rapid ignition and complete combustion.
Primary air is usually initially mixed with fuel resulting in rapid ignition of a flame. The primary air also serves to convey the fuel through the burner. Most burners are then designed to introduce additional secondary air as necessary at a later point to provide for complete combustion.
Oxides of nitrogen and carbon monoxide are gaseous pollutant products of the combustion of hydrocarbon fuels. Pollution level restrictions promulgated by the Environmental Protection Agency call for the reduction or elimination of these pollutants.
In particular, nitrogen oxide (NOx) emission regulations that are applied to combustion processes are becoming increasingly stringent. For example, California""s South Coast Air Quality Management District (xe2x80x9cSCAQMDxe2x80x9d) has promulgated regulations to limit the NOx emissions from burners operating with natural gas to a level of less than 25 parts per million on a volume basis (xe2x80x9cppmvxe2x80x9d, or simply referred to herein as xe2x80x9cppmxe2x80x9d), when corrected to a 3% oxygen level. Other states too are exploring, or have already passed, similar legislation.
In general, reducing pollutant emissions generated by way of fuel-burning processes can be accomplished in one of two ways: first, by selecting a fuel having the lowest overall level of pollutants, and second, by developing burning apparatuses and processes which can minimize the production and release of pollutants.
Combustion reactions can generally produce NOx via one of two mechanisms, depending on the type of fuel that is used. First, fuel NOx is produced from chemically bound nitrogen present in the fuel that is to be combusted. Second, thermal NOx is produced in high temperature flames by fixation from nitrogen and oxygen present in the combustion air. As a practical matter, depending on the nitrogen concentration present in the fuel, fuel NOx generation rates can be orders of magnitude greater than thermal NOx generation rates.
NOx emission may be limited to the thermal variety if natural gas (rather than coal or oil for instance) is employed as the fuel of choice, since clean natural gas does not comprise any nitrogen containing compounds. The generally accepted mechanism for thermal NOx formation can be described by the following reaction equations:
N2+O  less than = greater than NO+Nxe2x80x83xe2x80x83(1) 
O2+N  less than = greater than NO+Oxe2x80x83xe2x80x83(2) 
Additionally, it is generally known that the NOx generation rate can be decreased by cooling the temperature of a combustion flame in a burner. Further, a decrease in combustion flame temperature most significantly effects the production of thermal NOx. Also, NOx pollution reduction by way of a reduction in the combustion flame temperature is most effective when natural gas is the fuel of choice.
Current low NOx burners include post combustion or flue gas scrubbing mechanisms that typically involve a catalytic process that typically requires expensive add-ons. Also, metal fiber burners or ceramic heads can be constructed to lower emissions, but such devices tend to require high excess air levels (normally around 9 percent O2). This results in an increase in overall fuel consumption. Further, these and other current low pollutant burners/burner add-ons are often unreliable and can require significant servicing. Moreover, such burners/burner add-ons often yield poor flame density and shape, this can result in an unstable combustion process.
Incomplete combustion results in the gaseous combustion products containing a high percentage of CO, unburned hydrocarbons and carbonaceious materials. Complete combustion results in the oxidizing of such CO, hydrocarbons and carbonaceous materials into innocuous CO2. Ideally, complete combustion can take place under conditions (e.g., lower temperature) that will not result in nitrogen (again, present in fuel and air) being oxidized to form significant quantities of NOx.
Burner and boiler systems with burners are well known and commercially available. Generally, methods for reducing combustion emissions, combustion product discharge, and pollutants, are also known. These topics are discussed with varying degrees of particularity in U.S. Pat. Nos. 5,667,374, 5,195,883, 5,522,696, 4,659,305, 4,050,877, 4,013,499, and 3,955,909, the disclosures of which are incorporated by reference herein.
It would be desirable to have a low NOx emission burner that solves the aforementioned problems. More specifically, it would be desirable to have a low NOx emission burner that lowers the excess air requirements of current burners, reduces the NOx emissions to a level of less than 12 parts per million (ppm). Additionally, the burner would preferably reduce CO emission to a level of less than 50 ppm. Further, the preferred burner would achieve these emission levels reliably, consume less fuel while attaining better combustion efficiencies, all without requiring expensive add-on equipment or additional manufacturing and maintenance costs typically associated with other such low pollutant burners.
The present invention provides a low pollutant emission burner that overcomes the aforementioned problems, and does so in a fashion that is cost effective, efficient and adaptable to a variety of uses and configurations.
Hence, in accordance with one aspect of the invention, a low pollution emission burner system is provided, the burner system comprising: a fuel manifold comprising a housing, the housing defining an interior area comprising a first chamber and a second chamber; a first set of injectors for injecting a fuel from the first chamber, the injectors disposed radially inward from the fuel manifold; a second set of injectors for injecting the fuel from the second chamber into a stream of air to pre-mix the fuel and the air, the second set of injectors disposed radially inward from the fuel manifold; a third set of injectors for injecting the fuel, the third set of injectors located in an area defined by at least one of the first and the second set of injectors; and a refractory located downstream of the fuel manifold, the refractory comprising a plurality of channels for introducing air and combustion product into a combustion chamber, the combustion chamber located downstream of the refractory. The burner system can comprise a steam injector located upstream of the first set of injectors, the steam injector for injecting steam within the burner.
In accordance with another aspect of the invention, a method for reducing pollution emissions from a burner, the method comprising the steps of: providing a fuel manifold comprising a housing, the housing defining an interior area comprising a first chamber and a second chamber; providing a first set of injectors, a second set of injectors, and a third set of injectors within a burner system, the first set of injectors connected to, and disposed radially inward from, the first chamber, the second set of injectors connected to the second chamber and the third set of injectors located in an area defined by at least one of the first and the second set of injectors; introducing air and fuel into the burner; injecting the fuel from the first set of fuel injectors; injecting the fuel from the second set of fuel injectors into a stream of air to obtain pre-mixture of fuel and air; injecting the fuel from the third set of fuel injectors; igniting at least one of the fuel and the mixture of fuel and air from at least one set of fuel injectors to create a flame and a resulting combustion product; recirculating at least a portion of combustion product into the burner; and passing at least one of air and at least a portion of the combustion product through at least one channel in the refractory to a location downstream of the refractory.
Accordingly, the invention accomplishes a reduction in air pollution by reducing NOx emissions to a level of less than about 10 ppm, CO emissions to a level of less than about 50 ppm, in addition to reducing the hydrocarbon and particulate content of the exhaust gases from carbonaceous and hydrocarbon fuel burners. Preferably, these reductions are achieved without sacrificing efficiency by using O2 levels of between about 2.5 percent and about 3.5 percent. O2 levels can be reduced to about 2 percent depending on the application at hand.
The inventive burner shortens, cools and more evenly shapes the burner flame and provides good flame stability throughout the burner combustion range so as to minimize burner servicing costs and increase, for example, boiler (or other apparatus to which the burner is attached) life expectancy. Hence, the inventive burner system provides a cost-effective approach to reducing pollutant emissions.
The inventive burner effectuates a reduction in NOx production without adversely affecting the thermal combustion efficiency of the burner by using heat that is normally lost to the stack to, for example, preheat combustion air.
The burner can reduce air pollutants, for example NOx and CO. The burner preferably is readily adaptable to various types of apparatuses, for example, boilers (e.g., water and fire tube boilers). The burner can preferably be incorporated into new boilers or added to existing units. The burner can allow boiler installations to meet increasingly stringent air quality emission limitations.